BUG: Augmented arithmetic assignments will modify original series starting version 1.1.4

[nikita-ivanov]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

import pandas as pd

s = pd.Series([1, 2, 3])
s1 = s.iloc[1:]
s1 -= 4
s

# output
# 0    1
# 1   -2
# 2   -1
# dtype: int64

s = pd.Series([1, 2, 3])
s1 = s.iloc[1:]
s1 = s1 - 4
s

# output
# 0    1
# 1    2
# 2    3
# dtype: int64

Problem description

Starting with version 1.1.4 when applying either of +=, -=, *= or /= operators on a series s1 that was sliced from original series s, the change propagates to original series s. When using normal assignment operators s1 = s1 - 4 the problem is not present, which leads to inconsistent behavior.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 67a3d42
python : 3.7.9.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.18362
machine : AMD64
processor : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.None

pandas : 1.1.4
numpy : 1.17.2
pytz : 2020.4
dateutil : 2.8.0
pip : 20.3.3
setuptools : 51.0.0.post20201207
Cython : 0.28.5
pytest : 3.8.1
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.19.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.3.3
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.5.4
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[simonjayhawkins]

Thanks @nikita-ivanov for the report.

Starting with version 1.1.4 when applying either of +=, -=, *= or /= operators on a series s1 that was sliced from original series s, the change propagates to original series s.

This was changed in [d8c8cbb] REGR: inplace Series op not actually operating inplace (#37508) to fix a regression for DataFrame behaviour.

It looks like is also corrected a long standing bug/inconsistency with the series behaviour.

cc @jbrockmendel

[nikita-ivanov]

@simonjayhawkins thanks for the update. Do I see it correctly that the future behavior would the same as it is now for -= (and similar operations)? More precisely,

s = pd.Series([1, 2, 3])
s1 = s.iloc[1:]
s1 -= 4
s

# output
# 0    1
# 1   -2
# 2   -1
# dtype: int64

s = pd.Series([1, 2, 3])
s1 = s.iloc[1:]
s1 = s1 - 4
s

# output
# 0    1
# 1   -2
# 2   -1
# dtype: int64

[simonjayhawkins]

for the s1 = s1 - 4 case, the expected output remains as

# 0    1
# 1    2
# 2    3
# dtype: int64

When using normal assignment operators s1 = s1 - 4 the problem is not present, which leads to inconsistent behavior.

in Python, the a = a+1 is not always equivalent to a+=1.

Under the hood, Python calls 'special methods' for the various syntactical constructs. in this case __add__ and __iadd__

if the object, in this case a, implements __iadd__, that will be called. In the case of mutable sequences, a will change in place. However, when a does not implement __iadd__, the expression a+=1 has the same effect as
a = a+1. The expression a+1 will be evaluated first, producing a new object, which is then bound to a. In other words, the object bound to a may or may not change, depending on whether a implements __iadd__

In general, for mutable sequences, it is common for __iadd__ to be implemented and that += happens inplace. For immutable sequences, clearly there is no way for that to happen.

A pandas Series is a mutable sequence that implements __iadd__

[nikita-ivanov]

@simonjayhawkins thanks for clarifications. My concern is that prior to 1.1.4 the behavior was the same for both cases. And I did not find change of behavior information in the release notes. However, this change can cause some bugs (the case for me). I shall switch to s1 = s1 - 4, but it might be worthwhile to mention new behavior in the release notes.

[simonjayhawkins]

@nikita-ivanov I agree that a long standing behaviour has been changed without adequate notice.

We will await comment from @jbrockmendel for the way forward here.

I am reluctant to add a regression tag here as I think the new behaviour is more correct and we are not currently planning any more releases in the 1.1.x series.

[jbrockmendel]

I am reluctant to add a regression tag here as I think the new behaviour is more correct

Agreed.

[MarcoGorelli]

I am reluctant to add a regression tag here as I think the new behaviour is more correct

Agreed.

So, should the release notes be clarified, or can this be closed? @jbrockmendel